1. Field of the Invention
The invention relates to pipes and tubing having a wall structure composed of fiber reinforced polymer composite laminates.
2. Description of Related Art
Fiber reinforced plastic pipe (FRP pipe) is finding increased usage as piping in chemical plants as well as casing used in the drilling of oil and gas wells and casing and tubing for the transport of crude oil and natural gas up from the well source.
The advantage of FRP pipe over carbon steel pipe in oil/gas applications includes superior corrosion resistance, flexibility in achieving mechanical property design targets and improved fatigue resistance. FRP pipes are also of considerably lighter weight for a given wall thickness than their steel pipe counterparts.
FRP pipe designed for use in high pressure piping or casing such as crude oil pipelines and oil well tubing are generally prepared by impregnating a roving of filaments of a high strength material, such as continuous glass filaments, with a thermoset table resin composition, such as an epoxy resin, and winding the impregnated filaments back and forth onto a mandrel under tension to form a plurality of inter meshed filament windings. Filaments may be wound at an angle of 90xc2x0 to the pipe axis or at angles of 0xc2x0 to plus and minus about 90xc2x0 (+/xe2x88x9290xc2x0) with respect to the pipe axis. A helical filament winding pattern is formed when the winding angle is between 0xc2x0 and 90xc2x0 with respect to the longitudinal pipe axis. After a desired pipe wall thickness is achieved, the winding operation is discontinued, the resin is cured and the mandrel is extracted resulting in a cylindrical pipe having a fiber reinforced wall structure. FRP pipes of this type and their method of production are disclosed, e.g., in U.S. Pat. Nos. 2,843,153 and 5,330,807, the complete disclosures of which patents are incorporated herein by reference.
FRP pipe designed for use in onshore or offshore fossil fuel recovery must be constructed to withstand two basic forces to which it will be subjected. The first force is an outer radial load exerted along a vector normal to the pipe walls by fluids (oil or drilling muds) which are conveyed. under moderate to high pressure through the pipe, also known as the hoop load. The second force is an axial tensile load exerted along vectors parallel to the pipe axis and occasioned by the fluid pressure and/or the weight of a long string of coupled pipe sections suspended in the ground at the well bore and/or between the well bore and surface platform in offshore recovery operations. These strings are often suspended 3,000 to 10,000 feet (about 850 to 2800 meters), and thus must be able to carry a long term axial stress in excess of about 2500 pounds per square inch (or 2.5 ksi) occasioned during operation and when the pipe string is inserted and removed during the fossil fuel recovery process.
FRP pipe having maximum hoop strength can be designed if the reinforcing fiber is wound at an angle close to 90xc2x0 to the pipe axis, e.g., +/xe2x88x9270xc2x0 up to 90xc2x0. Conversely, maximum tensile strength is developed where the reinforcing fiber is applied at an angle close to 0xc2x0 to the pipe axis, e.g. +/xe2x88x9230xc2x0 down to 0xc2x0. However, pipe wound at or close to 90xc2x0 exhibits severe diminishment of axial tensile strength while pipe wound at or close to 0xc2x0 exhibits severe diminishment of hoop strength. Pipe wound at intermediate pipe axis angles between +/xe2x88x9230xc2x0 to +/xe2x88x9270xc2x0 (as disclosed in U.S. Pat. No. 2,843,153) generally compromises hoop and particularly axial strength and may be insufficiently strong for practical use in many fossil fuel recovery operations.
One technique for attempting to maximize both hoop and axial strength is to lay down the reinforcing fiber composite as separate laminate layers one atop another, each layer having the fibers disposed at different pipe axial angles designed to maximize the hoop or axial stress bearing properties of the pipe as well as minimize the coefficient of expansion of the composite pipe. An example of such a construction containing +/xe2x88x9220xc2x0 to +/xe2x88x9260xc2x0 fiber layers alternating with 90xc2x0 layers is disclosed in U.S. Pat. No. 5,330,807. Other similar layered laminates are disclosed in U.S. Pat. Nos. 4,728,224 and 4,385,644.
Laminates of this type comprising a plurality, e.g., 3 to 9, separate layers are generally designed for an optimization of hoop or axial stiffness and therefore do not take advantage of the anisotropy of unidirectional fiber composites. For instance, alternating a 0 and +/xe2x88x9270 degree lay-up does not take advantage of the maximum hoop strength of the +/xe2x88x9270 degree layer or the maximum axial strength of the 0 degree layer.
Also, composite laminates currently commercially available exhibit a serious deficiency which makes their use not cost effective in applications that generate even moderate pipe stress levels. Microcracking and delamination of the pipe wall structure at or near the pipe joints and/or along the pipe length provide a leak path for fluids, commonly referred to as xe2x80x9cweepingxe2x80x9d, which can occur at fluid pressures which can be 5 to 10 times less than the pipe short-term burst pressure. Intrusion of water into the pipe wall structure via these micro cracks can attack glass fiber surfaces and/or binder resin, leading to delamination and premature pipe failure.
Although microcracking can be mitigated by increasing the pipe wall thickness, this solution drives the composite pipe and tubing cost up as compared to that of carbon steel. The higher cost constitutes a barrier to the substitution of composite pipes and tubing for carbon steel in moderate to high (injection) pressure applications. Also, in down hole applications, the increased wall thickness prevents the use of composites where the diameter of the well bore is constrained, because of the cross-sectional area available for fluids to flow is smaller than that of carbon steel. The use of composites in these applications would require drill holes with larger diameter, and this gives rise to additional drilling costs.
The axial strength of composite pipe cannot be significantly increased by increasing the wall thickness. This limits composite down hole tubing, casing, and injection tubing to wells whose depth does not exceed about 5000 ft.
Accordingly, it is a primary object of this invention to provide layered composite FRP piping having acceptable hoop and axial strength which is more resistant to microcracking and delamination on the one hand and also has diminished wall thickness on the other hand such that the piping is compatible with carbon steel well bore/casing dimensions.
The present invention provides a fiber reinforced plastic pipe having a hollow tubular body with a wall structure formed from a plurality of layers, each layer containing fibers that may be the same for each layer or different, the fibers being fixed in a resin binder and oriented at a substantially fixed angle with respect to the longitudinal axis of the pipe, comprising: an outer axial load-bearing layer containing a plurality of first fibers, the first fibers ranging in thickness (diameter) from about 1 to less than 14 xcexcm and disposed at a substantially fixed angle ranging from 0xc2x0 to about +/xe2x88x9230xc2x0, and a second layer in fixed contact with the outer layer disposed radially inward of the outer layer, the second layer containing a plurality of second fibers disposed at a substantially fixed angle of greater than +/xe2x88x9230xc2x0, the second fibers ranging in thickness (diameter) from about 1 xcexcm to about 24 xcexcm.
The pipe is designed so that when male threaded joint sections are molded or cut at the outer wall surface of one or both ends of the pipe, the molded/cut threads extend into/onto the axial load bearing layer of the pipe such that this layer carries substantially all of the axial stress generated during the mechanics of fossil fuel recovery. This reduces the shear stress and axial strain mismatch between the axial load bearing layer and adjacent layer(s) which are designed to maximize the hoop strength of the pipe.
As used herein, substantially fixed means that the angle will only vary by the amount of accuracy in the filament winding equipment when a fixed angle is chosen. Typically, no more than plus or minus 5 degrees, preferably no more than plus or minus 2 degrees, and more preferably no more than plus or minus 1 degree of variation will be present. One skilled in the art recognizes that the lower the variation, the better. However, it is also recognized that slight variations are, in most cases, unavoidable.